1. Field of the Invention
This invention relates to a 1-phase energized disk-type brushless motor having a single position detecting element.
2. Description of the Prior Art
As various systems have been developed, brushless motors have been in demand, especially disk-type brushless motors suitable for use with such systems. Disk-type brushless motors can be used as disk-type brushless fan motors which are widely used in office machines and the like. In some applications, these disk-type brushless fan motors are required to be very inexpensive, small and very flattened, setting aside the rotational efficiency (naturally, however, the rotational efficiency must be higher than a particular level in order that they may be of practical use as such).
Those motors which meet the requirements best are 1-phase energized disk-type brushless motors which include a single armature coil and a single position detecting element. However, such a 1-phase energized disk-type brushless motor limited to the above construction cannot actually operate as a motor because it cannot be rotated continuously, although the magnet rotor can turn over a predetermined range. Otherwise, even if a motor having a single armature coil and a single position detecting element can rotate continuously, the single armature coil could not provide a sufficiently large turning force. Therefore, for a sufficient turning force, two or more armature coils must necessarily be provided.
A disk-type brushless motor having two armature coils for a stator armature normally requires two or more position detecting elements. In most cases, magnetoelectric transducers such as Hall elements or Hall ICs (integrated circuits) are used for the position detecting elements. However, since such position detecting elements are expensive, it is desirable for a motor to include, if possible, only one position detecting element in order that inexpensive, small disk-type brushless motors can be mass produced. However, such a motor having a single position detecting element has a drawback that it cannot start itself if the position detecting element detects a boundary between the N (north) pole and the S (south) pole of the magnet rotor, that is, a dead point, similarly as in a motor having a single coil as described above. In this way, a 1-phase motor has a dead point at an energization switching point at which the motor provides zero or no torque. Therefore, the 1-phase motor has a drawback that it cannot start itself if the rotor position upon starting the motor is just at a dead point.
Accordingly, a 1-phase motor is normally provided with a cogging generating magnetic member (an iron piece is used therefor) for generating a torque (cogging torque) in addition to a torque generated by an armature coil and a field magnet (rotor magnet) in order to eliminate such dead points to allow self-starting of the motor.
In a coreless motor, for example, following methods for generating a cogging torque are known. Referring first to FIG. 1, a 6-pole field magnet 2 having an alternate arrangement of the 6 north and south poles is mounted on a rotor yoke 1 in an opposing relationship to a stator yoke 5 with an air gap 4 left therebetween and with a pair of coreless armature coils 3 disposed in the air gap 4. In the motor of FIG. 1, the stator yoke 5 has at a face thereof opposing the field magnet 2 two inclined surfaces which thus define the complementarily inclined air gap 4. This method, however, has a drawback that the efficiency is relatively low because the air gap is relatively great.
Referring now to FIG. 2, another method is illustrated. In the motor of FIG. 2, a stator yoke 5 has no such inclined faces as provided on the stator yoke 5 of FIG. 1. Instead, an iron bar 6 is mounted on the stator yoke 5 and extends through each of a pair of coreless armature coils 3 disposed in a uniform air gap 4 defined by the stator yoke 5 and a field magnet 2 on a rotor yoke 1. According to this arrangement, a magnetic flux will appear as seen in FIG. 3 and hence the field magnet 2 will stop at a position in which the iron bars 6 are each opposed to the center of one of the N and S poles of the field magnet 2. Accordingly, if the armature coils 3 are located so as to produce a rotational torque in such a stopping position of the field magnet 2, a coreless motor which can start itself will be obtained.
However, the method as shown in FIG. 2 has a drawback that if the thickness of the iron bars 6 is increased in order to increase the cogging torque, the torque around the dead points will decrease because a magnetic flux 7 will act as shown in FIG. 4 around the dead points.